In recent years, wireless LAN and power-line communications (PLC), which do not need supplementary wiring for home networks, have started to become commonplace. However, when trying to add a new apparatus into such a home network, it is necessary to perform an initial period authentication for the connection (only once at the outset) with an apparatus that will serve as a base unit apparatus, and a new apparatus that will serve as a portable handset apparatus. Since these communication methods, such as wireless LAN and power-line communications (PLC), use open-source media and can experience interference between apparatuses, there is a possibility that a non-predetermined apparatus will connect by mistake in the case where mutual authentication is not performed correctly in the initial connection period for each apparatus.
An authentication scheme based on a common key is generally used for this kind of mutual authenticating method. For example, in the international standard for wireless LAN, IEEE 802.11i, an authentication scheme like the following is specified (see for example, Non-patent Document 1). FIG. 1 is a sequence diagram for this authentication scheme. After authentication is initialized, an authentication request 11 is sent to a wireless base station (AP) 2 from a station (STA) 1. Upon receiving the authentication request 11, the wireless base station (AP) 2 creates a challenge plaintext (CT) 12 and returns it to the station (STA) 1. The station (STA) 1 encrypts the received challenge plaintext 12 by its common key PSK, and returns a challenge cryptogram (PSK(CT)) 13 to the wireless base station (AP) 2. The wireless base station (AP) 2 decrypts the received challenge cryptogram 13 by its own common key PSK, and when the decrypted item matches the challenge inside the challenge plaintext 12 that was sent to the station (STA) 1, an authentication response (successful) 14 is returned to the station (STA) 1, and in the case where they do not match, an authentication response 14 (unsuccessful) is returned to the station (STA) 1. Provided here that the common key PSK of the station (STA) 1 and the common key PSK of the wireless base station (AP) 2 must be identical to the keys set or distributed beforehand.
A simple method is actually preferable to a complicated method for the setting and distribution method of the communication apparatus' common key on a home network. For example, methods such as storing a key inside a communication apparatus beforehand, or building a key generation procedure into a communication apparatus, so that a user may choose a corresponding key by simply entering a key code are conceivable. By choosing a key setting method like this, an input means for key settings which a communication apparatus must include, can be easily implemented at an extremely low cost.
[Non-patent Document 1] IEEE Std 802.11i-2004, Chapter 8, Page 39-Page 41
However, when the input means is streamlined in the setting or distribution method for the common key as above, the number of pattern functions which can be inputted decreases and the chosen key duplicates; as a result, the possibility increases that authentication between non-predetermined apparatuses, which happen to have the same common key, will be allowed to succeed.
Because avoiding mistaken authentication is not considered in the authenticating method in Non-patent Document 1 above, two apparatuses in the same interference range (for example, neighboring houses) which have chosen the same common key, and are using wireless LAN or power line communications, may mistakenly authenticate and connect to each other. In addition, as for mischief and deliberate attacks, when a mistaken authentication occurs merely by choosing the same common key, and a connection is made, it is possible to easily perform eavesdropping and identity theft against an apparatus in a neighboring house.
Therefore, the present invention has been conceived in view of the above problems, and an object of the present invention is to provide a communication apparatus and an authenticating method, which can prevent mistaken authentication during initial authentication in a connection with a communication apparatus.